1. Field of the Invention
The present invention relates to an internal peripheral edge type blade holding device for use in an internal peripheral edge type slicing machine used when a fragile material such as a silicone ingot is sliced into thin pieces each having a tickness of the order of hundreds of microns.
2. Description of the Related Art
In general, a thin piece such as a semiconductor wafer is manufactured by slicing an ingot of a semiconductor or the like by use of a very thin cutting blade. As a typical example of such cutting blade, there is available a doughnutshaped blade and, especially, there is often used a blade provided with an internal peripheral edge which can be contructed by adhesively attaching diamond ground particles to the internal peripheral edge of the blade. In such internal peripheral edge type blade, in order to slice out a thin piece with a high accuracy, the external peripheral side of the blade is tensioned up and held by a blade holding device by means of a predetermined tension so as to be able to give the blade a predetermined tensile force.
Referring now to FIG. 7, there is shown a side section view of an internal peripheral edge type blade holding device H which is conventionally used. In this figure, there is shown a spindle 1, a bowl-shaped bottom ring 2 is fixedly mounted to the upper portion of the spindle 1, and a tension ring 3 is mounted to the top portion of the bottom ring 2. The blade, which is here designated by B, is put between and held by the tension ring 3 and a top ring 4 which can be mounted onto the tension ring 3 by means of bolts or the like.
According to a conventional well known method, the tension of the blade B can be adjusted by a large number of blade set bolts (not shown) which are provided on the circumference of the top ring 4. Such tension adjustment method is based on the existence of differences among the degrees of tensile strength with respect to the rolling direction, that is, an anisotropic property inherent in the blade B. In other words, due to the fact that the doughnutshaped blade is produced by press punching a rolled, stripshaped thin plate, the blade has no directional property because it is circular in appearance, but, however, in the blade, inherently, there is produced a difference in tensile strength between in the rolling direction and a direction intersecting perpendicularly with the rolling direction. For this reason, in the above-mentioned blade tension adjustment method, the pressing forces of the set bolts on the top ring circumference are set to vary according to positions.
In other words, since, as mentioned above, the abovementioned anisotropic property is found to exist in between the directions ( x-x direction, y-y direction ) which intersect substantially with each other, for example, if the tensile strength in the x direction is greater than that in the y direction, the pressing force in the x direction may be set greater than that in the y direction.
However, if the pressing forces are set to be different according to the positions of the blade as mentioned above, then uneven stress loads are given to the whole holding device H, which, in the end, may result in the sway of the blade in cutting, breakage or damage of a sliced thin piece, varying thicknesses of the sliced thin pieces, and other similar unfavorable phenomena.
In FIGS. 8 and 9, there is shown a state of the abovementioned holding device H deformed due to the above-mentioned stress. Specifically, in these figures, a solid line is used to illustrate the shape of the blade holding device H before the stress loads are applied thereto, while a two-dot chained line is used to illustrate the shape of the blade holding device H after the stress loads are applied thereto.
In other words, in these figures, when the pressing force in the x direction side of the blade B is set greater than that in the y direction thereof as mentioned above, then a similar load, as the reaction force thereof, is produced in the blade holding device H. In this case, a tension head assembly 5, which is composed of the top ring 4 and tension ring 3 of the holding device, is displace, when viewed in the plane shape thereof, inwardly in the x direction and outwardly in the y direction thereof, so that the tension head assembly 5 turns into an elliptical shape, as shown by the two-dot chained line in FIG. 8. Such deformation of the tension head assembly 5 has an effect on the bowl-shaped bottom ring 2 as well. That is, because the tension head assembly 5 has a ring-like shape, it provides the elliptical shape when it is deformed, but, however, the bottom ring 2 cannot be deformed into such elliptical shape as mentioned above due to the existence of a bottom plate 6. For this reason, as shown in FIG. 9, in the x direction (the shorter diameter direction) of the bottom ring 2 the peripheral side of the bottom plate is curvedly deformed in the upward direction, while in the y direction (the longer diameter direction ) of the bottom ring 2 the peripheral side of the bottom plate is curvedly deformed in the downward direction. The deformation of the bottom plate as it is, has an influence on the tension head assembly 5. As described above, while the plane shape of the tension head assembly 5 is deformed into the elliptical form due to the stress loads, the side face shape of the tension head assembly 5 is deformed in an arc-like manner with the abovementioned bottom plate peripheral side as a fulcrum because the deformation of the bottom plate 6 has an influence thereon. That is, as shown in the illustrated embodiment, when viewed from the x direction, the blade surface is deformed in the upward direction from the normal level thereof, while, when viewed from the y direction, the blade surface is deformed in the downward direction, so that the whole blade surface provides an uneven shape like a swell. This disadvantageously results in the sway of the surface of the bottom plate 6 as well as in the sway of the edge of the blade in cutting.